I'm sorry to hear about your dog's diagnosis. Heartguard, unlike sentinel, is not only a preventative but can kill te larvae of the heartworms (the immature stage). However, if there are already adults present, they will likely be present and in full force in 2 years. I would first speak to the vet on what heartworm test was run (was it just checked for larvae under the microscope or was a test for adults done). If adults could be present, the treatment is ivermectin again (the main ingredient in heartguard), however, it is given in large doses and injected. This must be done at the vet's hospital as reactions to it can be severe, and if the dog was infested with them, to kill them is dangerous as well, because they can get trapped in a major artery, in which they live in (next to the heart).
So basically, speak to your vet about what test was done and if just giving heartguard each month is sufficient if adults are present, as likely it is not. It is safe to give the heartguard now before talking to him.
Hope this helps. Let me know if I can help further.
It looks like Christine isn't going to be on today and you have been waiting some time. Would you like some help or do you want to wait for Christine?